Nonwoven fabrics of thermoplastic resin fibers, typically polypropylene nonwoven fabrics, have excellent breathability, flexibility and lightweight properties and have recently been used widely in various applications. Thus, the nonwoven fabrics require specific properties in accordance with the applications, and demands have been placed on the improvements of such properties.
In particular, disposable diapers have recently come to be heavily used in emerging countries, most typically China, due to population growth and have good market potentials in these countries. On the other hand, an increase in CO2 emissions associated with the large consumption of disposable diapers is becoming a serious environmental problem. In order to suppress the worldwide increase in CO2 emissions, plant-derived materials have been studied. However, the results are unsuccessful in terms of quality, costs and productivity. Meanwhile, manufacturers of disposable diapers have worked on saving CO2 emissions by reducing the weight of nonwoven fabrics and packages, achieving only limited effects.
As an approach to substantially reducing the weight of nonwoven fabrics, nonwoven fabrics composed of hollow fibers have been proposed in various processes. For example, Patent Literature 1 proposes polypropylene nonwoven fabrics suitable as sanitary materials having a fiber diameter of not more than 20 μm and a hollowness of 5 to 70%. Table 1 in Patent Literature 1 describes a nonwoven fabric with a fiber diameter of 22.2 μm, a hollowness of 13% and a basis weight of 22.2 g/m2.
Further, Patent Literature 2 proposes nonwoven fabrics that include continuous hollow fibers of a propylene polymer having a ratio (Mz/Mw) of Z average molecular weight (Mz) to weight average molecular weight (Mw) in the range from 1.5 to 1.9. Example 1 of this literature describes a spunbonded nonwoven fabric with a fiber diameter of 21.5 μm, a hollowness of 28.5% and a basis weight of 30 g/m2.
Spunbonded nonwoven fabrics with excellent lightweight properties can be obtained according to the methods for the production of spunbonded nonwoven fabrics described in these patent literatures. In the case where the hollow fibers forming the spunbonded nonwoven fabrics are made finer to a fiber diameter of not more than 20 μm and a basis weight of not more than 20 g/m2 in order to further reduce the weight of the spunbonded nonwoven fabrics, however, it has been found that the obtainable nonwoven fabrics are so nonuniform that they cannot be suitably used as sanitary materials.